Pattern formation method

ABSTRACT

After flattening a surface of an underlying film that has pores or includes an organic material by treating the underlying film in a supercritical fluid, a resist film made of a chemically amplified resist material is formed on the underlying film whose surface has been flattened. Next, pattern exposure is performed by selectively irradiating the resist film with exposing light, and then, the resist film is developed after the pattern exposure, so as to form a resist pattern.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a pattern formation method foruse in fabrication processing or the like for a semiconductor integratedcircuit device.

[0002] In the fabrication processing for a semiconductor integratedcircuit device or the like, the size of a resist pattern (pattern width)formed by lithography technique is further refined in accordance withincrease in the degree of integration of semiconductor integratedcircuits, and accordingly, the aspect ratio of a resist pattern isremarkably increasing.

[0003] Also, the dielectric constant of an insulating film is desired tobe further lowered in accordance with improved performance ofsemiconductor devices. Therefore, use of a low dielectric insulatingfilm that has a lower dielectric constant than a generally used siliconoxide film, such as an insulating film having pores or including anorganic material, has been proposed.

[0004] Now, a conventional pattern formation method will be describedwith reference to FIGS. 6A through 6D.

[0005] First, a chemically amplified resist material having thefollowing composition is prepared: Base polymer: poly((methoxymethylacrylate) - (γ-butyrolactone   2 g methacrylate)) (wherein methoxymethylacrylate: γ-butyrolactone methacrylate = 70 mol %:30 mol %) Acidgenerator: triphenylsulfonium triflate 0.04 g Solvent: propylene glycolmonomethyl ether acetate   20 g

[0006] Next, an organic polymer made of aromatic hydrocarbon includingno fluorine (for example, SiLK manufactured by Hitachi Chemical Co.,Ltd. (with a dielectric constant of 2.65)) is deposited on a substrate 1so as to form a low dielectric insulating film 2. Thereafter, thechemically amplified resist material having the aforementionedcomposition is applied on the low dielectric film 2, and then, theresultant substrate 1 is annealed with a hot plate (not shown) at atemperature of 90° C. for 60 seconds. Thus, a resist film 3 with athickness of 0.4 μm is formed.

[0007] Next, as shown in FIG. 6B, pattern exposure is carried out byirradiating the resist film 3 with ArF excimer laser 5 through aphotomask 4 having a desired pattern.

[0008] Then, as shown in FIG. 6C, the resist film 3 is subjected topost-exposure bake (PEB) by annealing the substrate 1 with a hot plate(not shown) at a temperature of 105° C. for 90 seconds. Thus, an exposedportion 3 a of the resist film 3 becomes soluble in an alkalinedeveloper because an acid is generated from the acid generator thereinwhile an unexposed portion 3 b of the resist film 3 remains insoluble inan alkaline developer because no acid is generated from the acidgenerator therein.

[0009] Next, after the pattern exposure, the resist film 3 is developedwith an alkaline developer of a 2.38 wt % tetramethylammonium hydroxideaqueous solution for 60 seconds and is then rinsed with pure water for60 seconds. Thereafter, the resultant resist film 3 is dried. Thus, aresist pattern 6 with a pattern width of 0.11 μm is formed from theunexposed portion 3 b of the resist film 3 as shown in FIG. 6D.

[0010] The cross-sectional shape of the resist pattern 6 has, however, afooting shape as shown in FIG. 6D, and thus, the pattern shape isdefective.

[0011] The conventional pattern formation method shown in FIGS. 6Athrough 6D is employed for forming a positive resist pattern 6. In thecase where a negative resist pattern is formed, the resultant resistpattern has an undercut cross-sectional shape, and the pattern shape isalso defective.

[0012] When a resist pattern in a defective pattern shape is used foretching a film to be etched, the shape of the resultant pattern of theetched film is also defective, which disadvantageously lowers the yieldof semiconductor devices.

SUMMARY OF THE INVENTION

[0013] In consideration of the aforementioned conventional disadvantage,an object of the invention is forming a resist pattern in a good patternshape.

[0014] In order to achieve the object, the present inventors have made avariety of examinations on the cause of the defective shape of a resistpattern. As a result, it has been found that in the case where anunderlying film has pores or includes an organic material, the shape ofa resultant resist pattern formed thereon becomes defective.

[0015] Also, the reason why a resist pattern formed on an underlyingfilm having pores or including an organic material becomes defective hasbeen studied, resulting in finding the following phenomenon: When aresist film made of a chemically amplified resist material is formed ona low dielectric insulating film, such as a porous film having pores oran organic film having a rough surface because of an organic materialincluded therein, and is subjected to pattern exposure, an acidgenerated in an exposed portion of the resist film is incorporated inthe pores or recesses of the rough surface of the underlying film. As aresult, the amount of acid is so insufficient at the bottom of theexposed portion of the resist film that a catalytic reaction of the acidcannot be sufficiently carried out. Therefore, solubility in a developeris spoiled at the bottom of the exposed portion of a positive resistfilm or insolubility in a developer is spoiled at the bottom of theexposed portion of a negative resist film. As a result, the positiveresist pattern is in a defective footing shape and the negative resistpattern is in a defective undercut shape.

[0016] Accordingly, examination has been made on means for preventingthe acid generated in the exposed portion of the resist film from beingincorporated in the pores or the recesses of the underlying film. As aresult, it has been found that when the underlying film is treated in asupercritical fluid, the surface of the underlying film is flattened,namely, the pores or the recesses of the underlying film are reduced,and therefore, the amount of acid incorporated in the underlying filmcan be reduced.

[0017] The present invention was devised on the basis of theaforementioned findings, and specifically, the pattern formation methodof this invention includes the steps of treating, in a supercriticalfluid, an underlying film that has pores or includes an organicmaterial, whereby flattening a surface of the underlying film; forming aresist film made of a chemically amplified resist material on theunderlying film whose surface has been flattened; performing patternexposure by selectively irradiating the resist film with exposing light;and forming a resist pattern by developing the resist film after thepattern exposure.

[0018] The pores of the underlying film may be formed within or on theunderlying film.

[0019] In the pattern formation method of this invention, afterflattening the surface of the underlying film having pores or includingan organic material through a treatment performed in a supercriticalfluid, namely, after reducing pores or recesses on the underlying film,the resist film made of the chemically amplified resist material isformed on the underlying film. Therefore, an acid generated in anexposed portion of the resist film is minimally incorporated in thepores or the recesses of the underlying film. Accordingly, a catalyticreaction of the acid is not reduced at the bottom of the exposed portionof the resist film. As a result, a footing shape of a positive resistpattern and an undercut shape of a negative resist pattern can besuppressed, so as to form a resist pattern in a good cross-sectionalshape.

[0020] In the pattern formation method of this invention, the chemicallyamplified resist material preferably contains, in a base polymerthereof, an acid labile group of an ester group (represented by ChemicalFormula 1).

[0021] Thus, the resist film can be made stiff so as to attain a goodpattern shape.

[0022] In this case, the ester group is preferably a t-butyl group, at-butyloxycarbonyl group or an adamantyl group. Chemical Formula 2 belowrepresents an ester group having a t-butyl group, Chemical Formula 3represents an ester group having a t-butyloxycarbonyl group, andChemical Formula 4 represents an ester group having an adamantyl group.

[0023] In the pattern formation method of this invention, the chemicallyamplified resist material preferably includes an acid generator composedof an imide compound.

[0024] An imide compound has a property that an anion and a cationgenerated through irradiation with exposing light approach each other,and therefore, the apparent size of the generated acid is larger than anacid generated from another acid generator. Therefore, the acidgenerated from an imide compound is minimally deactivated because it isdifficult to be incorporated in the pores or recesses on the underlyingfilm.

[0025] In this case, the imide compound is preferably benzeneiminotosylate, naphthaleneimino tosylate, benzeneimino triflate,naphthaleneimino triflate or phthalimino triflate. Chemical Formula 5below represents benzeneimino tosylate (phthalimino tosylate), ChemicalFormula 6 represents naphthaleneimino tosylate, Chemical Formula 7represents benzeneimino triflate, Chemical Formula 8 representsnaphthaleneimino triflate and Chemical Formula 9 represents phthaliminotriflate.

[0026] In the pattern formation method of this invention, the step oftreating an underlying film in a supercritical fluid preferablyincludes, in the following order, sub-steps of flattening the surface ofthe underlying film in the supercritical fluid that is placed in asubcritical state by being kept at a temperature lower than a criticaltemperature and at a pressure higher than a critical pressure; changingthe subcritical state of the supercritical fluid to a supercriticalstate by heating the supercritical fluid placed in the subcriticalstate; and restoring the supercritical state of the supercritical fluidto a general state by lowering a pressure of the supercritical fluidplaced in the supercritical state.

[0027] When the underlying film is thus replaced with a supercriticalfluid placed in a subcritical state, namely, a supercritical fluid witha high density, a material of convexes on the underlying film is rapidlyreplaced with the supercritical fluid placed in a subcritical state soas to be released from the surface of the underlying film. Therefore,the surface of the underlying film is rapidly flattened.

[0028] Also, since the supercritical fluid placed in a subcritical statethat has been used for flattening the surface of the underlying film ischanged to a supercritical fluid placed in a supercritical state byheating and then is restored to a fluid in a general state by reducing apressure. Therefore, a supercritical fluid in a fluid state and asupercritical fluid in a gas state are never present at the same time,so that the surface treatment of the underlying film can be efficientlycarried out.

[0029] In the pattern formation method of this invention, the step oftreating an underlying film in a supercritical fluid preferably includesa sub-step of flattening the surface of the underlying film in thesupercritical fluid that is placed in a supercritical state by beingkept at a temperature higher than a critical temperature and at apressure higher than a critical pressure.

[0030] Thus, the surface of the underlying film can be easily flattened.

[0031] In the pattern formation method of this invention, thesupercritical fluid is preferably a supercritical fluid of carbondioxide.

[0032] Thus, the supercritical fluid can be easily and definitelyobtained.

[0033] In the pattern formation method of this invention, thesupercritical fluid is preferably allowed to flow.

[0034] Thus, the surface material that has been replaced with thesupercritical fluid and is dissolved in the supercritical fluid isdischarged to the outside together with the flowing supercritical fluid,so that the surface of the underlying film can be efficiently flattened.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIGS. 1A, 1B and 1C are cross-sectional views for showingprocedures in a pattern formation method according to Embodiment 1 ofthe invention;

[0036]FIGS. 2A and 2B are cross-sectional views for showing otherprocedures in the pattern formation method of Embodiment 1;

[0037]FIGS. 3A, 3B and 3C are cross-sectional views for showingprocedures in a pattern formation method according to Embodiment 2 ofthe invention;

[0038]FIGS. 4A and 4B are cross-sectional views for showing otherprocedures in the pattern formation method of Embodiment 2;

[0039]FIG. 5 is a diagram for explaining respective states of asupercritical fluid; and

[0040]FIGS. 6A, 6B, 6C and 6D are cross-sectional views for showingprocedures in a conventional pattern formation method.

DETAILED DESCRIPTION OF THE INVENTION

[0041] Embodiment 1

[0042] A pattern formation method according to Embodiment 1 of theinvention will now be described with reference to FIGS. 1A through 1C,2A and 2B.

[0043] First, an organic polymer made of aromatic hydrocarbon includingno fluorine (for example, SILK manufactured by Hitachi Chemical Co.,Ltd. (with a dielectric constant of 2.65)) is deposited on a substrate10, so as to form a low dielectric insulating film 11 corresponding toan underlying film to be treated. Thus, the low dielectric insulatingfilm 11 is made of an organic polymer, and therefore, the low dielectricinsulating film 11 has a rough surface.

[0044] Then, the low dielectric insulating film 11 is placed in achamber 12. Thereafter, a supercritical fluid 14 of carbon dioxide (CO₂)(which is placed in a supercritical state by being kept at a temperatureof 40° C. and at 80 atmospheric pressure) is supplied from a cylinder 13into the chamber 12 for 30 minutes, and the supercritical fluid 14contained in the chamber 12 is discharged to the outside by a dischargepump 15. It is noted that the critical temperature of carbon dioxide is31.0° C. and the critical pressure of carbon dioxide is 72.9 atmosphericpressure.

[0045] In this manner, a material of recesses of the rough surface ofthe low dielectric insulating film 11 is replaced with the supercriticalfluid 14 and then is discharged to the outside of the chamber 12together with the supercritical fluid 14, and therefore, the roughnesson the surface of the low dielectric insulating film 11 is reduced. As aresult, the recesses on the surface of the low dielectric insulatingfilm 11 are reduced.

[0046] Thereafter, the low dielectric insulating film 11 whose surfacehas been flattened is moved to the outside of the chamber 12.

[0047] Furthermore, a positive chemically amplified resist materialhaving the following composition is prepared: Base polymer:poly((methoxymethyl acrylate) - (γ-butyrolactone   2 g methacrylate))(wherein methoxymethyl acrylate: γ-butyrolactone methacrylate = 70 mol%:30 mol %) Acid generator: triphenylsulfonium triflate 0.04 g Solvent:propylene glycol monomethyl ether acetate   20 g

[0048] Next, as shown in FIG. 1B, the chemically amplified resistmaterial having the aforementioned composition is applied on the lowdielectric insulating film 11 whose surface has been flattened, andthen, the resultant substrate 10 is annealed with a hot plate (notshown) at a temperature of 90° C. for 60 seconds. Thus, a resist film 16with a thickness of 0.4 μm is formed.

[0049] Then, as shown in FIG. 1C, pattern exposure is carried out byirradiating the resist film 16 with ArF excimer laser 18 emitted from anArF excimer laser exposure machine with numerical aperture NA of 0.60through a photomask 17 having a desired pattern.

[0050] Next, as shown in FIG. 2A, the resist film 16 is subjected topost-exposure bake (PEB) by annealing the substrate 10 with a hot plate(not shown) at a temperature of 105° C. for 90 seconds. Thus, an exposedportion 16 a of the resist film 16 becomes soluble in an alkalinedeveloper because an acid is generated from the acid generator thereinwhile an unexposed portion 16 b of the resist film 16 remains insolublein an alkaline developer because no acid is generated from the acidgenerator therein.

[0051] As described above, since the recesses on the surface of the lowdielectric insulating film 11 formed below the resist film 16 arereduced, the acid generated in the exposed portion 16 a of the resistfilm 16 is minimally incorporated in the recesses on the surface of thelow dielectric insulating film 11. Therefore, a catalytic reaction ofthe acid can be satisfactorily carried out in the exposed portion 16 aof the resist film 16.

[0052] Next, after the pattern exposure, the resist film 16 is developedwith an alkaline developer of a 2.38 wt % tetramethylammonium hydroxideaqueous solution for 60 seconds and then is rinsed with pure water for60 seconds. Then, the resultant resist film 16 is dried. Thus, a resistpattern 19 with a pattern width of 0.11 μm is formed from the unexposedportion 16 b of the resist film 16 as shown in FIG. 2B.

[0053] In this case, since the catalytic reaction of the acid issatisfactorily carried out in the exposed portion 16 a of the resistfilm 16, the resist pattern 19 formed from the unexposed portion 16 b ofthe resist film 16 can be in a good rectangular cross-sectional shapefree from a footing shape.

[0054] In Embodiment 1, the surface treatment of the low dielectricinsulating film 11 is performed in the supercritical fluid of carbondioxide placed in a supercritical state. Instead, the surface treatmentmay be performed in a supercritical fluid of carbon dioxide placed in asubcritical state. This method will be now be described with referenceto FIG. 5.

[0055] First, the low dielectric insulating film 11 is held, for 40seconds, in a supercritical fluid of carbon dioxide that is placed in asubcritical state by being kept at a temperature lower than the criticaltemperature (Tc), for example, at 28° C. and at a pressure higher thanthe critical pressure (Pc), for example, at 80 atmospheric pressure.Thus, the material of the recesses of the rough surface of the lowdielectric insulating film 11 is replaced with the supercritical fluidof carbon dioxide in a subcritical state. In this case, the material ofthe recesses is efficiently replaced with the supercritical fluid thathas a high density because of its subcritical state, and therefore, therecesses on the surface of the low dielectric insulating film 11 arerapidly reduced.

[0056] Next, the supercritical fluid of carbon dioxide in a subcriticalstate is heated to a temperature higher than the critical temperature(Tc), for example, to 40° C. while keeping the pressure higher than thecritical pressure (Pc), thereby changing the subcritical state of thesupercritical fluid to a supercritical state. Thereafter, while keepingthe temperature higher than the supercritical temperature (Tc), thepressure higher than the supercritical pressure (Pc) is restored to theatmospheric pressure, thereby changing the supercritical state of thesupercritical fluid to a subcritical state. Thereafter, the temperaturehigher than the supercritical temperature (Tc) is restored to the roomtemperature, so as to change the supercritical fluid placed in asubcritical state to a general fluid.

[0057] In this manner, it is possible to avoid a state where asupercritical fluid in a liquid state and a supercritical fluid in a gasstate are present at the same time, and therefore, the surface treatmentcan be satisfactorily performed on the low dielectric insulating film11.

[0058] Modification of Embodiment 1

[0059] A pattern formation method according to a modification ofEmbodiment 1 will now be described. This modification is different fromEmbodiment 1 in the chemically amplified resist material alone, andhence, the chemically amplified resist material alone will be hereindescribed. Specifically, the chemically amplified resist material usedin the modification has the following composition: Base polymer:poly((2-methyl-2-adamantyl acrylate) -   2 g (γ-butyrolactonemethacrylate)) (wherein 2-methyl-2-adamantyl acrylate: γ-butyrolactonemethacrylate = 70 mol %:30 mol %) Acid generator: naphthaleneiminotosylate 0.04 g Solvent: propylene glycol monomethyl ether acetate   20g

[0060] In this chemically amplified resist material, the base polymerhas an adamantyl group as an acid labile group of an ester group.Instead, the base polymer may have a t-butyl group or at-butyloxycarbonyl group.

[0061] Also, in the chemically amplified resist material,naphthaleneimino tosylate is used as an acid generator made of an imidecompound. Instead, the acid generator may be benzeneimino tosylate,benzeneimino triflate, naphthaleneimino triflate or phthaliminotriflate.

[0062] Furthermore, although the acid labile group of an ester group andthe acid generator made of an imide compound are both used in thismodification, merely one of them may be used. Also in that case, asimilar satisfactory effect can be attained.

[0063] Embodiment 2

[0064] A pattern formation method according to Embodiment 2 of theinvention will now be described with reference to FIGS. 3A through 3C,4A and 4B.

[0065] First, siloxane doped with carbon atoms (with a dielectricconstant of 2.5) is deposited on a substrate 20, so as to form a lowdielectric insulating film 21 corresponding to an underlying film to betreated. Thus, the low dielectric insulating film 21 is made ofcarbon-containing siloxane, and therefore, the low dielectric insulatingfilm 21 has a rough surface.

[0066] Then, the low dielectric insulating film 21 is placed in achamber 22. Thereafter, the low dielectric insulating film 21 issubjected to a surface flattening treatment with a supercritical fluid23 of carbon dioxide (CO₂) (which is placed in a subcritical state bybeing kept at a temperature of 20° C. and at 80 atmospheric pressure)for 30 minutes.

[0067] In this manner, in the chamber 22, a material of recesses of therough surface of the low dielectric insulating film 21 is efficientlyreplaced with the supercritical fluid that has a high density because ofits subcritical state, and therefore, the recesses on the surface of thelow dielectric insulating film 21 are rapidly reduced. Thereafter, thelow dielectric insulating film 21 whose surface has been flattened ismoved to the outside of the chamber 22.

[0068] Furthermore, a negative chemically amplified resist materialhaving the following composition is prepared: Base polymer: poly(vinylphenol)   6 g Crosslinking agent: 0.12 g2,4,6-tris(methoxymethyl)amino-1,3,5-s-triazine Acid generator:phthalimino triflate 0.02 g Solvent: propylene glycol monomethyl etheracetate   30 g

[0069] Next, as shown in FIG. 3B, the chemically amplified resistmaterial having the aforementioned composition is applied on the lowdielectric insulating film 21 whose surface has been flattened, andthen, the resultant substrate 20 is annealed with a hot plate (notshown) at a temperature of 100° C. for 90 seconds. Thus, a resist film24 with a thickness of 0.4 μm is formed.

[0070] Then, as shown in FIG. 3C, pattern exposure is carried out byirradiating the resist film 24 with KrF excimer laser 26 emitted from aKrF excimer laser exposure machine with numerical aperture NA of 0.68through a photomask 25 having a desired pattern.

[0071] Next, as shown in FIG. 4A, the resist film 24 is subjected topost-exposure bake (PEB) by annealing the substrate 20 with a hot plate(not shown) at a temperature of 120° C. for 90 seconds. Thus, an exposedportion 24 a of the resist film 24 becomes insoluble in an alkalinedeveloper because an acid is generated from the acid generator thereinwhile an unexposed portion 24 b of the resist film 24 remains soluble inan alkaline developer because no acid is generated from the acidgenerator therein.

[0072] As described above, since the recesses on the surface of the lowdielectric insulating film 21 formed below the resist film 24 arereduced, the acid generated in the exposed portion 24 a of the resistfilm 24 is minimally incorporated in the recesses on the surface of thelow dielectric insulating film 21. Therefore, a catalytic reaction ofthe acid can be satisfactorily carried out in the exposed portion 24 aof the resist film 24.

[0073] Next, after the pattern exposure, the resist film 24 is developedwith an alkaline developer of a 2.38 wt % tetramethylammonium hydroxideaqueous solution for 60 seconds and then is rinsed with pure water for60 seconds. Then, the resultant resist film 24 is dried. Thus, a resistpattern 27 with a pattern width of 0.12 μm is formed from the exposedportion 24 a of the resist film 24 as shown in FIG. 4B.

[0074] In this case, since the catalytic reaction of the acid issatisfactorily carried out in the exposed portion 24 a of the resistfilm 24, the resist pattern 27 formed from the exposed portion 24 a ofthe resist film 24 can be in a good rectangular cross-sectional shapefree from an undercut.

[0075] A supercritical fluid has a higher density at a lower temperaturewhen the pressure is constant. Accordingly, when the surface treatmentis performed in the supercritical fluid of carbon dioxide that is placedin a subcritical state by being kept at a temperature of 20° C. and at80 atmospheric pressure as in Embodiment 2, the material of concaves ofthe rough surface of the low dielectric insulating film 21 isefficiently replaced with the supercritical fluid 23 in a subcriticalstate, and therefore, the surface treatment of the low dielectricinsulating film 21 is rapidly carried out.

[0076] Preferably, as in Embodiment 1, the supercritical fluid of carbondioxide placed in a subcritical state by being kept at a temperaturelower than the critical temperature (Tc) and at a pressure higher thanthe critical pressure (Pc) is heated to a temperature higher than thecritical temperature (Tc) while keeping the pressure higher than thecritical pressure (Pc), so as to change the subcritical state of thesupercritical fluid to a supercritical state, and thereafter, whilekeeping the temperature higher than the supercritical temperature (Tc),the pressure higher than the supercritical pressure (Pc) is restored tothe atmospheric pressure, so as to change the supercritical state of thesupercritical fluid to a subcritical state, and then, the temperaturehigher than the supercritical temperature (Tc) is restored to the roomtemperature, so as to change the supercritical fluid placed in asubcritical state to a general fluid.

[0077] In this manner, it is possible to avoid a state where asupercritical fluid in a liquid state and a supercritical fluid in a gasstate are present at the same time as shown in FIG. 5, and therefore,the surface treatment can be satisfactorily performed on the lowdielectric insulating film 21.

[0078] Although carbon dioxide is singly used as the supercritical fluidin Embodiments 1 and 2, a small amount of organic solvent such asalcohol, hydrocarbon, ether or carboxylic acid, may be added to carbondioxide as an entrainer. Thus, the replacement of alcohol with thesupercritical fluid is accelerated.

[0079] Also, although the supercritical fluid of carbon dioxide (with acritical temperature of 31.0° C. and a critical pressure of 72.9atmospheric pressure) is used in Embodiments 1 and 2, a supercriticalfluid of water (H₂O) (with a critical temperature of 374.2° C. and acritical pressure of 218.3 atmospheric pressure) or a supercriticalfluid of ammonia (NH₃) (with a critical temperature of 132.3° C. and acritical pressure of 111.3 atmospheric pressure) may be used instead.However, carbon dioxide can be easily placed in a supercritical statebecause its critical temperature and critical pressure are lower thanthose of the other fluids.

What is claimed is:
 1. A pattern formation method comprising the stepsof: treating, in a supercritical fluid, an underlying film that haspores or includes an organic material, whereby flattening a surface ofsaid underlying film; forming a resist film made of a chemicallyamplified resist material on said underlying film whose surface has beenflattened; performing pattern exposure by selectively irradiating saidresist film with exposing light; and forming a resist pattern bydeveloping said resist film after the pattern exposure.
 2. The patternformation method of claim 1, wherein said chemically amplified resistmaterial contains, in a base polymer thereof, an acid labile group of anester group.
 3. The pattern formation method of claim 2, wherein saidester group is a t-butyl group, a t-butyloxycarbonyl group or anadamantyl group.
 4. The pattern formation method of claim 1, whereinsaid chemically amplified resist material includes an acid generatorcomposed of an imide compound.
 5. The pattern formation method of claim4, wherein said imide compound is benzeneimino tosylate,naphthaleneimino tosylate, benzeneimino triflate, naphthaleneiminotriflate or phthalimino triflate.
 6. The pattern formation method ofclaim 1, wherein the step of treating an underlying film in asupercritical fluid includes, in the following order, sub-steps of:flattening the surface of said underlying film in said supercriticalfluid that is placed in a subcritical state by being kept at atemperature lower than a critical temperature and at a pressure higherthan a critical pressure; changing the subcritical state of saidsupercritical fluid to a supercritical state by heating saidsupercritical fluid placed in the subcritical state; and restoring thesupercritical state of said supercritical fluid to a general state bylowering a pressure of said supercritical fluid placed in thesupercritical state.
 7. The pattern formation method of claim 1, whereinthe step of treating an underlying film in a supercritical fluidincludes a sub-step of flattening the surface of said underlying film insaid supercritical fluid that is placed in a supercritical state bybeing kept at a temperature higher than a critical temperature and at apressure higher than a critical pressure.
 8. The pattern formationmethod of claim 1, wherein said supercritical fluid is a supercriticalfluid of carbon dioxide.
 9. The pattern formation method of claim 1,wherein said supercritical fluid is allowed to flow.